In non-mechanical printers represented, for example, by electrophotographic printers, an image composed of individual dots is generated with the aid of a plurality of light elements of an optical character generator. The light elements can be, for example, light-emitting diodes which are brightly gated on by means of pulse-shaped signals of a particular frequency for generating the image. Since, in the case of a plurality of LEDs on an optical character generator, it cannot be expected that the light energy emitted by the LEDs is always the same over the operating time and from LED to LED because of the component spread and aging, it is necessary to calibrate optical character generators and during this process to measure the radiant energy, that is to say the radiant power multiplied by the duration of each individual light-emitting diode and to calibrate it in dependence on this measuring process, that is to say to adapt it with respect to the light energy. To automate this calibration, each individual LED is activated with a variable pulse-shaped signal in dependence on the desired radiant energy. In this process, the possible variations are the magnitude of the activating pulse-shaped signal and the variation of the duty ratio of the activating current, which is used with preference.
Before, for example, the duty ratio can be correctly adjusted, the emittable light energy must be determined at a predetermined duty ratio for each individual LED by means of a measuring arrangement so that the necessary variation of the duty ratio can then be carried out by means of a correction device. The light energy in each case emitted for each individual LED is measured at a constant duty ratio by means of a moving measuring arrangement in which a photoelement is conducted by means of a motor-driven slider over the strip of light-emitting diodes, that is to say over each individual LED. Such an automatic calibration arrangement for the character generator of an electrophotographic printer is known from U.S. Pat. No. 4,780,731.
The electrophotographic printer described there contains an exposure energy correction device for the optical character generator exhibiting a strip of light-emitting diodes. When a calibration routine is called up, the light-emitting elements are automatically calibrated by the fact that a photoelement detects the radiant power emitted by the light-emitting element and supplies it in the form of electrical signals to a control device coupled to the light-emitting element. The program-controlled device then allocates to each light-emitting element an individual operating time and stores this in a switching-time memory. As a result, each light-emitting element later supplies this radiant energy when operated.
To measure the radiant energy, an extremely flat photoelement component is used which, in the form of a slider, moves past each individual LED during the measuring process. Since it must be reliably encountered by the radiation of each light-emitting diode, it also has a very large area (approximately 1 cm.times.0.5 cm). However, very flat photoelements which, nevertheless, have a large area, have the disadvantage that, on the one hand, they only have low sensitivity and, on the other hand, have relatively large unwanted electrical capacity which makes it more difficult to measure short light flashes. In addition, the conditions can vary significantly when the photoelements are changed.
If rectangular pulses are used as light pulses, the electrical response pulses emanating from the photoelement are greatly changed.
If the radiant power or the radiant energy of light-emitting semiconductor elements such as light-emitting diodes or the like are determined by means of large-area photoelements via individual light pulses or light flashes, the results achieved are inaccurate and in some circumstances may be afflicted with interference.